BEIJING (Reuters) - An invasion of unidentified worms has forced 50 herdsmen and their families from their grassland homes, taking 20,000 head of livestock with them, in northwest China's Xinjiang region, state news agency Xinhua said Friday.

The worms are packed up to 3,000 per square meter and chew through the grasslands like lawnmowers, leaving only brown soil in their wake, Xinhua said.

The agency described it as the worst plague in three decades in Usu, about 280 km (175 miles) west of the Xinjiang capital Urumqi.

Local experts could not identify the 2-cm (1 inch) long, thorny green worm with black stripes and samples had been sent to Xinjiang Agricultural University, Xinhua said.

"The pasture was green a week ago. But now the worms are creeping around, and they even come into my house. I have to sweep them out several times an hour," Xinhua quoted one herdsman as saying.

Xinjiang has in the past used chickens, ducks and other birds to fight locusts, which are also a menace on the grasslands, but so far they have shown little interest in the pesky worms.

One local official said the worms might be moth larvae that have flourished in the relatively warm winter and plenty of rain.